A compensating method of the kind referred to above is primarily utilized in the area of microlithography. Masks are illuminated by laser radiation and imaged onto a silicon wafer by means of an optical system. Such systems must image structural sizes of 0.5 to 1.0 .mu.m with high precision and changes of environmental parameters such as temperature and air pressure lead to a noticeable deterioration of the imaging in such systems.
Apparatus for microlithography are used for producing integrated circuits. Such apparatus are generally operated in climatized rooms at constant temperature. An additional stabilization of air pressure is not provided since the size of the apparatus would require a very complex and costly pressure chamber which does not appear to be economically justifiable. For this reason, the production of integrated circuits with such apparatus is either discontinued when certain air pressure limits are exceeded or one of the following methods are utilized:
(1) Rinsing the image forming objective and/or the object and image chamber with gases and specifically with (a) helium since the change of index of refraction of this gas reacts relatively little to environmental changes, or (b) with an oxygen/nitrogen mixture with the adjustment of the mixture ratio being utilized for air pressure compensation; and, PA1 (2) Readjusting of the object and image planes so that the linear magnification remains constant.
Rinsing with gas has the disadvantage that special preconditions as to the seal tightness of the area which is rinsed with gas must be satisfied and thereby a relatively large effort is required. The readjustment means an interruption of production and thereby causes a reduction of the usable operating time. Furthermore, this control can be carried out only within narrow limits since it is accompanied by noticeable distortions.